1. Field of the Invention
This invention relates to an antiskid brake system for a vehicle which suppresses braking force so that an excessive braking force is not applied to the wheels of the vehicle during braking, and more particularly to such an antiskid brake system in which the braking pressure is periodically increased and reduced on the basis of wheel speeds detected by wheel speed sensors.
2. Description of the Prior Art
Recently many cars are provided with an antiskid brake system for preventing wheel lockup to prevent skid of the wheels during braking. The antiskid brake system generally comprises a solenoid control valve provided in the brake lines to control the braking pressure, wheel speed sensors for detecting the rotational speeds of the respective wheels, and a control unit which controls the solenoid control valve to suppress the braking force when it detects wheel lockup on the basis of the wheel speeds detected by the wheel speed sensors. That is, the control unit calculates the acceleration or deceleration of each of the wheels on the basis of the wheel speed detected by the wheel speed sensor and calculates the slip ratio of the wheel from the wheel speed and a pseudo vehicle speed estimated on the basis of the wheel speeds, and controls the solenoid control valve to reduce the braking pressure when the deceleration of the wheel speed falls below a predetermined value or the slip ratio reaches a predetermined value and to increase the braking pressure when the rotational speed increases due to reduction of the braking pressure and the acceleration of the wheel speed reaches a predetermined value. Such a series of braking pressure control (will be referred to as the antiskid control, hereinbelow) is repeated, for instance, until the vehicle stops. By such an antiskid brake system, lockup or skid of the wheels during sharp braking can be prevented and the vehicle can be stopped in a short distance without depriving directional stability.
In the antiskid brake system of this type, the wheel speed sensor measures the rotational speed of the wheel by detecting the rotational angle of the wheel per unit time and accordingly, the detecting accuracy tends to deteriorate as the wheel speed lowers. Therefore, at a low vehicle speed, especially at a very low vehicle speed just before the vehicle stopping, the vehicle speed cannot be precisely estimated and at the worst, the brake system operates as a normal brake system (a brake system having no antiskid control function).
In the antiskid brake system disclosed in Japanese Unexamined Patent Publication No. 4(1992)-201772, the antiskid control is terminated when the wheel speed detected by the wheel speed sensor falls below a reference value, and the barking pressure is increased at a rate which is reduced as the friction coefficient of the road surface estimated on the basis of the wheel speed reduces. With this arrangement, deterioration of controllability at a low vehicle speed, where the detecting accuracy of the wheel speed sensor is low, can be avoided and at the same time, on a road surface having a low friction coefficient such as a frozen road surface (will be referred to as "low .mu. road surface", hereinbelow), the vehicle can be stably stopped without wheel lockup since the braking pressure is gradually increased while on a road surface having a high friction coefficient such as a dry pavement surface, the vehicle can be surely stopped.
However the antiskid brake system has the following drawbacks due to the fact that the antiskid control is terminated when the wheel speed detected by the wheel speed sensor falls below a reference value.
That is, for example, assuming that the brake pedal is pushed down while the vehicle running a road surface having a high friction coefficient such as a dry pavement, the antiskid control is continued until just before the vehicle stopping though the road condition is not such that gives rise to skid of the wheels and the braking pressure can be unexpectedly reduced, for instance, just before the vehicle stops, which results in a stopping distance longer than that expected by the driver. This is due to the fact that the antiskid control is not terminated until the wheel speed falls below the reference value.
However if the reference value is set high, lockup or skid of the wheels cannot be effectively prevented on a low .mu. road surface.